USING L1 INSERTS TO DETERMINE THE FACTORS THAT AFFECT MUTATION RATE The neutral DNA mutation rate (i.e., the accumulation of base substitutions in the absence of natural selection) is a fundamental biological parameter. Interestingly, it varies within and between chromosomes, but despite considerable study, this variation remains largely unexplained. We implemented an experimental system to determine in vivo whether DNA repair can induce mutations in flanking DNA and found that it does. In particular, the repair intermediates generated from preformed normally occurring DNA mispairs on an SV40-based episome were vulnerable at a low but statistically significant frequency to an APOBEC-mediated error-prone process. SiRNA knockdowns showed that components of both the base excision repair and mismatch repair pathways, or factors that can interact with these pathways (e.g., PCNA and ATR), and TpC-preferring APOBEC deaminases are all required for mutagenesis which produces mutations similar to those typical of the mutator phenotypes in various cancers. Thus normally error-free DNA repair processes can be turned into mutators providing a heretofore unexpected source of genetic changes that underlie disease, aging and evolutionary change. We have now are developing methods using reverse Chip to isolate mismatch-containing DNA molecules to determine which factors induce binding of APOBEC deaminases. We also are using deep sequencing to comprehensively examine the sequence context of the mismatch-containing DNA that affects binding of the DNA glycosylases that initiate the repair process.